One of known location estimation technologies calculates a first measurement position of a vehicle based on radio positioning signals received from positioning satellites. The technology also calculates a second measurement position of the vehicle using autonomous navigation with a vehicle speed sensor and a gyro sensor. Then, the technology estimates the location of the vehicle based on the first and second measurement positions.
Deterioration of reception of the radio positioning signals, multipath propagation of the radio positioning signals, or other similar factors due to blocking objects that block the radio positioning signals may cause errors in the first measurement position calculated based on the radio positioning signals. In addition, repeatedly updating the second measurement position of the vehicle using autonomous navigation may cause errors in the updated second measurement positions to be accumulated, resulting in the total value of the errors being larger. This may make it difficult to accurately estimate the location of the vehicle based on the first and second measurement positions.
From this viewpoint, Japanese Patent Application Publication No. 2008-157636 discloses a technology, referred to as a published technology; the published technology calculates the distance between a vehicle and a target object, such as a guardrail or a road sign, based on image data of the target object captured by a camera. Then, the published technology estimates the location of the vehicle based on the location of the target object in map data, and the calculated distance between the vehicle and the target object; this location of the vehicle will be referred to as map-based location of the vehicle.
In particular, the published technology corrects the estimated location of the vehicle based on the first and second measurement positions using the map-based location of the vehicle.
On the other hand, controlling a vehicle while the vehicle keeps a travelling lane of a road requires accurate estimation of the location of the vehicle in the width direction of the road.